GetTempFileNameA function (fileapi.h)

Article
02/09/2023

Creates a name for a temporary file. If a unique file name is generated, an empty file is created and the handle to it is released; otherwise, only a file name is generated.

Syntax

UINT GetTempFileNameA(
  [in]  LPCSTR lpPathName,
  [in]  LPCSTR lpPrefixString,
  [in]  UINT   uUnique,
  [out] LPSTR  lpTempFileName
);

Parameters

[in] lpPathName

The directory path for the file name. Applications typically specify a period (.) for the current directory or the result of the GetTempPath2 function. The string cannot be longer than MAX_PATH–14 characters or GetTempFileName will fail. If this parameter is NULL, the function fails.

[in] lpPrefixString

The null-terminated prefix string. The function uses up to the first three characters of this string as the prefix of the file name. This string must consist of characters in the OEM-defined character set.

[in] uUnique

An unsigned integer to be used in creating the temporary file name. For more information, see Remarks.

If uUnique is zero, the function attempts to form a unique file name using the current system time. If the file already exists, the number is increased by one and the functions tests if this file already exists. This continues until a unique filename is found; the function creates a file by that name and closes it. Note that the function does not attempt to verify the uniqueness of the file name when uUnique is nonzero.

[out] lpTempFileName

A pointer to the buffer that receives the temporary file name. This buffer should be MAX_PATH characters to accommodate the path plus the terminating null character.

Return value

If the function succeeds, the return value specifies the unique numeric value used in the temporary file name. If the uUnique parameter is nonzero, the return value specifies that same number.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

The following is a possible return value.

Return value	Description
ERROR_BUFFER_OVERFLOW	The length of the string pointed to by the lpPathName parameter is more than MAX_PATH–14 characters.

Remarks

The GetTempFileName function creates a temporary file name of the following form:

<path>\<pre><uuuu>.TMP

The following table describes the file name syntax.

Component	Meaning
<path>	Path specified by the lpPathName parameter
<pre>	First three letters of the lpPrefixString string
<uuuu>	Hexadecimal value of uUnique

If uUnique is zero, GetTempFileName creates an empty file and closes it. If uUnique is not zero, you must create the file yourself. Only a file name is created, because GetTempFileName is not able to guarantee that the file name is unique.

Only the lower 16 bits of the uUnique parameter are used. This limits GetTempFileName to a maximum of 65,535 unique file names if the lpPathName and lpPrefixString parameters remain the same.

Due to the algorithm used to generate file names, GetTempFileName can perform poorly when creating a large number of files with the same prefix. In such cases, it is recommended that you construct unique file names based on GUIDs.

Temporary files whose names have been created by this function are not automatically deleted. To delete these files call DeleteFile.

To avoid problems resulting when converting an ANSI string, an application should call the CreateFile function to create a temporary file.

In Windows 8 and Windows Server 2012, this function is supported by the following technologies.

Technology	Supported
Server Message Block (SMB) 3.0 protocol	Yes
SMB 3.0 Transparent Failover (TFO)	Yes
SMB 3.0 with Scale-out File Shares (SO)	Yes
Cluster Shared Volume File System (CsvFS)	Yes
Resilient File System (ReFS)	Yes

Examples

For an example, see Creating and Using a Temporary File.

Note

The fileapi.h header defines GetTempFileName as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.

Requirements

Requirement	Value
Minimum supported client	Windows XP [desktop apps \| UWP apps]
Minimum supported server	Windows Server 2003 [desktop apps \| UWP apps]
Target Platform	Windows
Header	fileapi.h (include Windows.h)
Library	Kernel32.lib
DLL	Kernel32.dll

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